The present invention generally relates to apparatus and methods of producing heat sinks for lighting fixtures having temperature-sensitive components such as LEDs; particularly outdoor lighting fixtures or lighting fixtures in corrosive environments. More specifically, the present invention relates to apparatus and methods of producing, in a rapid, consistent, and cost-effective manner, LED lighting fixtures with a plurality of high aspect ratio heat sink fins—where “aspect ratio” refers to the ratio of fin height to the spacing between the midpoint of two adjacent fins—in a manner that produces full penetration at the weld joint.
In the current state of the art in lighting design, there is a push to move away from more traditional light sources in favor of light-emitting diodes (LEDs). LEDs offer instant on-off capability and no hazardous materials (unlike some metal halide sources), pose no threat of UV or IR exposure (unlike some fluorescent sources), have excellent color rendering (unlike some low pressure sodium sources), and can achieve long operating life (unlike some incandescent sources). LEDs can assume a number of sizes, shapes, components, and configurations, but generally speaking an “LED” refers to the combination of a substrate, an emitter (also referred to as a die or a chip), a phosphor, and a primary lens (also referred to as a primary optic). More generally, LEDs have an emitting side that emits light that has been filtered (via the phosphor) and at least preliminarily shaped (via the primary lens); LEDs have a thermal transfer side which dissipates heat away from the temperature-sensitive components of the LED (e.g., primary lens, phosphor, junction between electrical power means and the die); and the two are on generally opposite sides of the LED, and are generally equally important in realizing the assumed benefits of LEDs.
When lighting design seeks to integrate LEDs in high demand applications such as sports lighting—namely, applications that require driving LEDs near or past their manufacturer-recommended operating conditions to achieve a total light output, a number of operating hours, a target junction temperature, and/or to offset the high capital cost of LEDs versus traditional sources—a number of steps may need to be taken on both sides of the LED. On the emitting side of the LED it is not uncommon to have secondary and tertiary reflectors, lenses, visors, baffles, louvers, or filters (collectively “optics”) to ensure precise beam control, minimize glare, and maximize efficiency (both in transmission and in beam utilization). On the thermal transfer side it is not uncommon to have passive and active cooling techniques to ensure a desired efficacy (i.e., a desired luminous output (Im) for a given power input (W)); or, taking a different approach, to ensure junction temperature of the LED does not exceed some critical temperature—where “critical” is determined empirically, from explicit testing, by the manufacturer, or otherwise. Another provision made (primarily on the thermal transfer side, though optics could play a role) when integrating LEDs in any lighting design—and particularly when integrating LEDs into a high demand lighting application—is a suitable heat sink. Suitability of an LED heat sink, and methods of forming such, present challenges in the current art of LED lighting design; some such challenges are presently discussed.
First and foremost, a relatively uninterrupted thermal transfer path (also referred to as a thermal dissipation path) must exist from the thermal transfer side of the LED to the exterior of the lighting fixture housing the LED or to some other location (e.g., forced air conduit system, internal void elsewhere in the fixture) for a heat sink to be suitable—this is common knowledge. To that end it is not uncommon for fixture design to include a number of thermally conductive components in physical contact with the LEDs, and with each other. An LED may be mounted to a board, the board may be mounted to an aluminum substrate, the aluminum substrate may be mounted to an aluminum housing, and the aluminum housing may include or be affixed to a number of heat sink fins (e.g., to increase surface area of the heat sink). Other times the housing may be vented or aerodynamically designed so to permit beneficial wind flow—all examples of passive cooling techniques. More active techniques (e.g., forced fluid flow over components) are also possible. Sometimes alternative materials (e.g., silver substrates, sheet metal heat sink fins) are used.
Once an adequate thermal dissipation path is established for one or more operating conditions, a lighting designer is confronted with the challenge of how to produce the heat sink in a repeatable, consistent, and (preferably) cost-effective manner. Consider the above example where heat from an LED is drawn through a board, substrate, housing, and heat sink fins. Simply machining fins and housing collectively (i.e., integrally) from stock (e.g., 1000 series aluminum alloy) results in a final product on the order of 10% of the initial material—quite simply, a waste that is not economically sustainable. Extruding fins and housing collectively (i.e., integrally) from stock exceeds the tooling capacity (i.e., the pressure before tooling breakage) for known extrusion techniques for high aspect ratio fins (a limitation of around 15:1). A lighting designer must choose between lower aspect ratio heat sink fins—which may not result in a suitable heat sink—or may need to look at a simple housing to which heat sink fins are affixed. Of course, in the current state of the art, there are issues with the latter. Traditional welding methods (e.g., metal inert gas (MIG) welding) do not result in full penetration; namely, do not fully bond a fin to the housing along the entire length of the fin though the entire thickness of the fin. A gap (even if only on order of a few thousandths of an inch) is present on the non-welded side of the fin, and can act as a focal point for crevice corrosion (or in cases of dissimilar metals, galvanic corrosion); a similar, but much larger, gap is at the end of each fin and presents a similar issue. Even when corrosion may not be an issue, said gap is unsightly, compromises the structural integrity of the fin (e.g., in high wind conditions), adversely impacts thermal transfer, and may interfere with other finishing steps (e.g., anodizing, painting). Attempts to fill the gap via soldering, brazing, or using some other method with filler material having adequate flow properties does not adequately seal the gap, or does so but with inadequate thermal transfer properties, too many additional or time-consuming steps to be economically feasible, or with inadequate structural properties. Fillet welds using heat sink fins with a machined knife edge in combination with a backer plate still result in the aforementioned gap—and in cases where spacing between heat sink fins is on the order of a fraction of an inch, is not possible. Attempts to join fins from the back side (i.e., the surface which would eventually bond to the base or substrate)—see, for example, U.S. Publication No. 2013/0175019—do not provide penetration through the entire thickness of the part on the weld side and does not address the gap between fins on the fin side.
Simply put, close spacing of heat sink fins is often necessary in high demand lighting applications, as are very tall heat sink fins, resulting in a high aspect ratio (e.g., on the order of or greater than 18:1); again, because the LEDs are being driven near or past some capacity and heat must be drawn away as quickly and effectively as possible. High aspect ratio heat sink fins either cannot be made at all using some traditional joining/forming methods, or cannot be made if one expects full penetration at the weld joint (i.e., between the fin and the housing/substrate along the entire length and thickness of the fin). Even in instances where full penetration is not required and one could use a traditional joining/forming method (e.g., MIG weld a heat fin to a housing), often tens (if not several tens) of heat sink fins are needed for each high demand LED lighting fixture. It is inconceivable even the most skilled welder could weld each fin consistently and rapidly over long periods as may be needed in a production setting. So it can be seen that while traditional forming and joining methods exist and could, in some circumstances, be used to bond many closely spaced, high aspect ratio heat sink fins to a substrate such as a fixture housing, doing so often results in a lack of penetration at the weld, is subject to incongruities, and is not cost-effective in high production (if even possible).
The need to manage thermal characteristics of LEDs—regardless of the motivation—is fervent and constant. While a number of lighting design techniques could be used, heat sinks are and will likely remain a primary feature of said techniques. While a complex and substantial heat sink may, on paper, provide a suitable thermal dissipation path, manufacturing a complex and substantial heat sink is another matter—particularly when one considers producing such consistently, rapidly, and cost effectively as is needed in a production setting. Lighting design is pushing the operating limits of LEDs, and is pushing the limits of state-of-the-art joining, forming, and manufacturing techniques associated with LED heat sinks—to the point that there appears to be no suitable options in the art that satisfies all the aforementioned needs. Thus, there is room for improvement in the art.